1. Field of the Invention
The present invention generally relates to power converters, and more specifically, the present invention relates to a switching control circuit for flyback power converters.
2. Description of the Related Art
FIG. 1A shows a conventional flyback power converter. The power converter comprises a bridge rectifier 35, an input capacitor 36, a controller 90, a transformer 10, a power transistor 20, a detection resistor 31, a rectifier 40, an output capacitor 45, a secondary feedback circuit. The transformer 10 includes a primary winding NP, a secondary winding NS, and an auxiliary winding NA. The secondary feedback circuit comprises a resistor 52, a zener diode (served as a voltage regulator) 60 and an opto-coupler 50. The bridge rectifier 35 converts an alternating current (AC) input voltage VAC to a pulsating direct current (DC) input voltage VIN. This input voltage VIN is further filtered by the input capacitor 36 to become a nearly direct current input voltage VIN with only relatively small ripple. The controller 90 generates a switching signal SW coupled to switch the transformer 10 via the power transistor 20. The rectifier 40 and the output capacitor 45 are coupled to the secondary winding NS of the transformer 10 for generating an output voltage VO of the power converter. The switching signal SW is generated in response to a feedback signal VFB for regulating the output voltage VO of the power converter. The feedback signal VFB is coupled to an output of the power converter via the secondary feedback circuit. The feedback signal VFB is correlated with the output voltage VO of the power converter. The input capacitor 36 is utilized to store the energy and provide a minimum input voltage VIN to guarantee a proper operation of the power converter. Without the filtering operation of the input capacitor 36, the pulsating direct current input voltage VIN will result in higher output line ripple and none-linearly power converting operation. Furthermore, the input capacitor 36 is generally occupies significant space of the power converter. Whenever the input capacitor 36 can be eliminated, the advantages to the power converter will be longer life, smaller size, and greatly reduced manufacturing cost.
Therefore, a controller capable of regulating an output voltage of the power converter without the need of the input capacitor is desired by the industries.